An optical transmission system typically includes an optical transmitter connected to an optical receiver by an optical link that can consist of a single fiber or of a plurality of fiber sections interconnected by optical amplifiers and/or more complex coupling media, for example switches based on optical gates, guides and couplers.
A transmitter includes an optical signal source whose function is to modulate an optical carrier wave as a function of the information to be transmitted. The same link can convey a plurality of signals carried by carrier waves with different wavelengths. The resulting transmission system is known as a wavelength division multiplex (WDM) system.
The modulation technique most widely used at present is amplitude modulation. The modulation formats generally adopted are the NRZ and RZ formats. They code binary information by varying the power of the carrier wave between a low level corresponding to extinction of the wave and a high level corresponding to a maximum optical power. Those variations of level are triggered at times imposed by a clock of fixed period T, which defines successive time cells allocated to the binary data to be transmitted. By convention, the low and high levels usually represent the binary values “0” and “1”, respectively.
The amplitude modulation technique, often referred to as amplitude-shift keying (ASK), has the advantage of being relatively easy to implement using proven optical components, but has the disadvantage of being sensitive to chromatic dispersion, however. Although there are devices for compensating chromatic dispersion, such as dispersion compensating fibers (DCF), the compensation obtained is often imperfect, especially for compensating WDM channels distributed across a wide spectral band.
For this reason a new modulation format that is less sensitive to chromatic dispersion has been proposed. The optical transmission method using this format is known as phase-shaped binary transmission (PSBT) and is described, for example, in European Patent Application EP-A-0 792 036, filed Feb. 17, 1997, published Aug. 27, 1997, and the corresponding U.S. Pat. No. 5,920,416 granted Jul. 6, 1999.
To transmit, that method subjects a carrier wave to an optical phase shift with an absolute value of the order of 180° within each cell that corresponds to a logic “0” and that precedes or follows a cell containing a logic “1”. At the receiving end, the received optical signal can be converted into an electrical signal in the same way as an NRZ format modulated signal, i.e. using a simple photodetector.
In addition to improving resistance to chromatic dispersion, PSBT modulation also has the property that the spectrum of the resulting signal is half the width of that of NRZ modulation, which is advantageous for dense WDM transmission, in which the spectral gap between WDM channels is narrow.
PSBT modulation nevertheless has limitations as soon as non-linear optical phenomena become significant. This is the case, for example, if the optical power of the signal is increased in order to increase transmission distance. It is then found that, for a given value of the cumulative chromatic dispersion over the link, the quality of the received signal is degraded to a degree that depends strongly on the wavelength of the optical carrier of the signal. The system therefore becomes less resistant to chromatic dispersion, which limits the possibilities of transmitting WDM signals distributed across a wide spectral band over long distances without intermediate amplification.
Another situation in which this problem arises is that in which transmission is effected via optical coupling media including components that are non-linear even at low powers, for example optical gates based on semiconductor optical amplifiers. This latter situation is typically encountered in networks equipped with switches using that kind of optical gate.
A solution to the problem of a PSBT signal being degraded by the Kerr effect (phase self-modulation, phase or gain cross-modulation) is proposed in European Patent Application EP-A-0 975 107, filed Jul. 20, 1998 and published Jan. 26, 2000. The solution consists of using PSBT modulation modified to introduce a transient “chirp” whose sign and optimum value depend in particular on the cumulative dispersion in the link and on non-linear effects. However, producing the optimum transient “chirp” is no simple matter, and can further have the effect of widening the spectrum of the optical signal, which makes PSBT modulation less advantageous for dense WDM transmission.
Another way to circumvent some non-linear phenomena is to choose modulation techniques which maintain a constant optical power of the signal. This is the case with optical phase or frequency modulation. Frequency-shift keying (FSK) has the disadvantage that the resulting signal is sensitive to chromatic dispersion, however. Phase-shift keying (PSK) is less sensitive to chromatic dispersion but requires the phase of the signal to be perfectly stable for correct detection at the receiver. However, a variant known as differential phase-shift keying (DPSK) has the advantage of reducing this latter constraint. PSK or DPSK modulation nevertheless leads to a spectral width of the signal that is the same as with the NRZ modulation format, and thus less favorable than PSBT modulation for dense WDM transmission.